Conventional free-space optical spectrometers usually rely on the dispersion properties of diffractive elements, such as gratings, to separate optical frequencies in the far-field. However, in order to achieve high spectral resolution, the spectrometer typically has a very large size or a small input aperture that spatially constricts the input light. Therefore, there can be a trade-off between resolution, size, and “light-gathering capability” (also referred to as étendue), which is proportional to the effective area of the aperture and the square of the numerical aperture.
One way to overcome the above constraints uses on-chip spectrometers, which can have lateral dimensions on the order of hundreds of microns and are very high resolution. But these on-chip spectrometers tend to suffer from low étendue due to their small input apertures.
Another way uses many filters to spectrally resolve the input signal. For example, one can use narrow-band resonant filters to achieve high resolution, or use broadband filters and employ spectral reconstruction techniques to resolve features smaller than the bandwidth of the filters.
A third approach that may address the trade-off between throughput and resolution for conventional diffractive spectrometers is to replace the small input aperture with a so-called “coded aperture,” which allows for an increase in throughput. But this approach usually also includes solving an inverse computational problem to construct the spectrum, which can be complex.